Effective ways to paint textures
If you want to use it, GIMP is a great alternative to Photoshop with a lot of the same tools and features for free. From there you could probably follow a Photoshop tutorial relatively well like this one.

I don't do it as much any more, but when I did texture painting, it worked really well for me to have a texture paint window open on one side of my display and an image editor on the other (see this post). Doing this, you can get some really good detail as long as you have a good set of UVs. I never even had to use GIMP or Photoshop.